1. Field of the Invention
The present invention relates generally to a material structure prediction apparatus capable of predicting the structure of a material that exhibits phase transformation due to variation in temperature, a product manufacturing method and a material structure prediction method.
2. Description of the Related Art
In recent years, there has been a demand to improve the quality of materials used for industrial products in view of environmental protection and energy saving. Further, it is known that such material features as tensile strength, yield stress, expansion, etc., are closely associated with material structure at the microscopic level. In view of this, the material feature of a product has come to be created by operating a condition after hot processing or a cooling condition after the hot processing, thereby altering temperature change curve used as an index for a change in temperature applied to materials.
Yet further, when a material obtained after cooling and polishing is observed using an electron back-scattering diffraction (EBSD) device as an application apparatus of a scanning type electron microscope, the crystal orientation of each crystal grain in a precipitated phase (new phase) can be obtained. A definite relationship exists between the crystal orientation of a mother phase and that of the precipitated phase. It is known that the crystal orientation and grain boundary of the mother phase can be reconstructed using the definite relationship (see non patent document 1). Also, researches have been conducted in association with a nucleation rate and a growth rate used for formulation of progress of diffusion transformation during cooling (see non patent document 2).
As a temperature control apparatus, an apparatus is disclosed in which the quantity of heat generated when a rolled material is subjected to phase transformation is predicted using a phase transformation heat generation model, and a winding temperature is controlled to coincide with a predetermined temperature target value with the transformation heat quantity compensated for (see patent document 1).
To establish appropriate manufacturing conditions for acquiring desired material properties, it is necessary to repeat test production. However, the process of repeating test production involves much cost and labor. In the field of iron and steel materials, a structure material-quality prediction technique for predicting the structure and quality of a material as a product have been developed for a hot rolling process. The structure material-quality prediction technique is used to properly control a manufacturing condition, such as a temperature variation curve, in order to obtain desired material properties.
In existing structure material-quality prediction models, it is assumed that the material structure is uniform. Accordingly, no consideration is given to differences or variations in crystal grain size and shape in the material structure. Actually, however, each crystal grain differs in size and shape, and variation therein may sometimes be great, depending upon manufacturing conditions. These factors may well change the quality of the material.
Further, solid-phase transformation in a cooling process of a material from a hot state includes two phenomena, i.e., (i) crystal nucleation and (ii) growth of created crystal grains. Regarding the latter (crystal grain growth), an analysis, method called a multi-phase-field (MPF) method is known. For instance, in single-crystal growth, a simulation method is disclosed in which a change in grain shape due to grain growth is calculated using the multi-phase-field method (see patent document 2). Regarding the former (nucleation), however, merely a simple consideration that, for example, initial nucleation positions are manually imparted at the start of calculation is given in the conventional multi-phase-field method, irrespective of the fact that the position and time of an actual nucleation are determined from the structure of a mother structure and temperature variation. Further, although patent document 2 considers nucleation caused by distortion energy due to dislocation in a single crystal, the application of the method disclosed therein to a polycrystal metal has limitations, because, for example, it is known that in a polycrystal metal such as steel, nucleation may easily occur in the vicinity of the grain boundary of a mother phase structure.
The conventional multi-phase-field method is insufficient in quantitative prediction accuracy since it does not sufficiently consider nucleation behaviors, although it can obtain a qualitative tendency of variation due to grain growth for difference in crystal grain size and shape and these variations in a material structure. Consequently, it is difficult to apply the disclosed analysis method to a case where quantitative evaluation, such as prediction of material quality after cooling, is needed.
As described above, by the conventional multi-phase-field method, it is difficult for use in the quantitative evaluation of, for example, material quality to predict the structure of a material that exhibits phase transformation as a result of temperature variation.